This article points to this census press release, but it looks like the system for getting the raw numbers changed sometime between that release and now. There is still a graph where you can see the <16 age bracket just barely holding onto >50% White, and according to the article and not the census page 2007 on is minority White.

For a more specific percentage, this Goldman Sachs pdf says "by the year 2020, 50.2% of children under the age of 18 are expected to be part of a minority race or ethnic group", although it was published in 2015.